U.S. Patent Application Publication No. 2004/0150361 (hereinafter referred to as “US 2004/0150361 A1”) has the object of providing an electromagnetic suspension device for a vehicle in which, while an active control is carried out on a control-objective input force, input forces other than the control-objective force can be handled passively, and which enables not only simplification of the active control, but also improvement of energy efficiency (see paragraphs [0003] to [0005]).
In order to achieve this object, according to US 2004/0150361 A1, an electromagnetic suspension device for a vehicle (see abstract, FIG. 3) is equipped with an electromagnetic actuator 4, which is interposed in parallel with a spring element 7 between a sprung mass and an unsprung mass, and is driven by an electric motor 8. A motor controller 17 is configured to calculate a displacement input to the electromagnetic actuator 4, and to control the electric motor 8 so as to produce an optimal damping force responsive to the displacement input. An electrical damping element (an electric resistor 20 or an electric resonance circuit 21), which passively generates a damping force by dynamic braking of the electric motor 8 with respect to a displacement input from the unsprung mass, is connected in parallel with the electric motor 8 to a control circuit for the electric motor 8 (see abstract, FIG. 4, and FIG. 7).
Among such electrical damping elements, the electric resonance circuit 21 (FIG. 7) exhibits a resonance frequency, which is coincident with the resonance frequency of the unsprung mass (for example, 10 to 20 Hz), and includes a resistor R, a coil L, and a capacitor C (see paragraph [0049]). Consequently, it is possible to achieve both an effective active control with respect to the control-objective input force within a low frequency region including the sprung mass resonance frequency, and an effective passive control with respect to the control-objective input force in the vicinity of the unsprung mass resonance frequency (see paragraph [0065], FIG. 9). The active control is a current control for the electric motor 8, which is implemented with the object of providing a low frequency vibration control including an attitude control for the vehicle (see paragraph [0041]).